1. Field of the Invention
The present invention relates to a PON (Passive Optical Network) based on a WDM (Wavelength Division Multiplexing)/SCM (Sub-Carrier Multiplexing) scheme, and more particularly to a termination device for use in a PON system based on a WDM/SCM scheme, which can effectively use a WDM/SCM-based multi-link (i.e., multiplexing link) simultaneously with being compatible with a conventional Ethernet system, and can guarantee a transmission sequence between a transmission node and a reception node.
2. Description of the Related Art
There has recently been developed a PON (Passive Optical Network) system for satisfying a user-requested high frequency band for use in a digital communication service. The PON system is a subscriber distribution network for connecting a communication connection path requested between the Ethernet network and a subscriber to an optical wavelength provider using a passive optical element, and includes a variety of terminal devices for connecting the passive optical element to either a subscriber or an Ethernet network. A terminal device connected to the subscriber is called an ONT (Optical Network Terminal), and the other terminal device connected to the Ethernet network is called an OLT (Optical Line Terminal).
Basically, the PON system combines a plurality of ONTs with each other using an optical splitter (also called an optical distributor) so that a link ranging from the optical splitter to one OLT is shared with a plurality of subscribers. Therefore, there must be newly developed a method for multiplexing the link shared between the optical splitter and the OLT in an upstream service for transmitting data in the range from the subscribers to the Ethernet network.
There are a variety of such multiplexing methods, for example, a TDM (Time-Division Multiplexing) scheme and an active switching scheme (also called a Home-Run scheme). In the case of downstream transmission, the TDM scheme controls the ONT to perform photoelectric conversion so that it filters data using either an address or an additional ID (e.g., an EPON's LLID) of packets. In the case of upstream transmission, the TDM scheme controls each ONT to transmit an upstream packet over a timeslot having a predetermined time interval at a fixed time. A PON system using the aforementioned TDM scheme is called a TDM-PON system.
The active switching scheme (i.e., the Home-Run scheme) controls an active node ranging from an OLT to an ONT to classify physical links using a point-to-point access scheme, and uses the classified physical links. In this case, the active node must recognize a corresponding port using the photoelectric conversion process, and must perform electric-to-optical conversion such that it can perform an optical transmission operation in the range from a source to either the OLT or the ONT. This network using the aforementioned active switching scheme is called an active network.
The TDM-PON system has advantages in a variety of areas, for example, installation, extensibility, and economic efficiency of optical cables. There is no need for the TDM-PON system to require additional control operations in the case of downstream transmission, but it can perform a high-speed transmission operation. However, the TDM-PON system complicates an upstream transmission process, encounters a TDM delay in packet transmission, and allows two hierarchical packets to be processed by the ONT for every subscriber to be higher than a link speed between the OLT and the ONT, such that it may increase processing capacity instead of a packet reception rate.
The active network has advantages in that it enables individual ONTs of users to be independent of each other and performs upstream/downstream transmission services using the same method, however, it has disadvantages in that it is worse than the TDM-PON system in a variety of areas, for example, installation, extensibility, and economic efficiency of optical cables and must perform photoelectric conversion and electric-to-optical conversion operations in a subscriber distribution/access field.
In order to solve the aforementioned problems, as shown in FIG. 1, a WDM(Wavelength Division Multiplexing)/SCM(Sub-Carrier Multiplexing)-PON system for constructing a communication link requested between an OLT 14 and a plurality of ONTs 11 in the form of a multi-link in association with the WDM/SCM scheme is increasingly developed.
The WDM/SCM-PON system shown in FIG. 1 adapts an AWG (Arrayed Wavelength Grating) acting as a passive element and an optical splitter/combiner 12 to physically interconnect the ONTs. A multi-link between the ONTs 11 and the OLT 14 is composed of a plurality of wavelengths and a plurality of sub-carriers as shown in FIG. 2.
Referring to FIG. 2, a first ONT (ONT1) is assigned to a first channel of a wavelength #1 (i.e., Sub-carrier #1), a second ONT (ONT2) is assigned to a second channel of the wavelength #1 (i.e., Sub-carrier #2), and an N-th ONT(ONT N) is assigned to first and second channels of the other wavelength #2 (i.e., Sub-carriers #1 and #2). In this case, a subscriber's frame can be classified by MAC address information. In more detail, if a specific ONT assigned to a corresponding MAC address uses a single SCM channel, it can distinguish a wavelength and an SCM channel from each other, such that the OLT 14 can determine an output link and transmit a frame over the determined output link. Although the ONT assigned to the corresponding MAC address uses a multiplexing SCM channel, the OLT 14 classifies the multiplexing SCM channel using a single link and internally processes the classified multiplexing SCM channel, resulting in a switching to the output link.
The WDM/SCM scheme guarantees an independent connection configuration between the OLT 14 and the ONT 11 using a wavelength and a sub-carrier in a physical layer, and simplifies a control protocol for allocating/guaranteeing bandwidths, differently from the TDM scheme, resulting in increased applicability to a true commercial-use network.
However, the WDM/SCM PON system includes a plurality of SCM channels in the range of a single wavelength, and connects the ONT to the OLT in the form of a point-to-point access over the SCM channels. The WDM/SCM PON system requires a method for maintaining a prescribed frame sequence in transmission/reception modes, classifies traffic data to be transmitted over a link in the IEEE 802.3 link aggregation standard into a few conversation units, and guarantees the prescribed frame sequence in the conversation units. Therefore, provided that there are a small number of classifiable conversation units or an unbalanced bandwidth among the conversation units is formed, the WDM/SCM PON system cannot effectively use a desired link.
In order to solve the aforementioned problems, there has been newly proposed a variety of methods for maintaining a frame transmission sequence, for example, a padding method, a division method, and a tagging method. The padding method for transmitting a frame received from an upper layer using 1518 octets indicative of a maximum Ethernet transmission length is far from efficiency in traffic data of a short frame length, and must control a receiver to extract a padding part. The division method capable of forming a constant frame length divides a frame length into several units instead of padding the frame length, and transmits the divided frame units each having the same length. The division method adds a header having new division information to the divided frame units, transmits the divided frame units each having the header, and controls a reception end to reconstruct reception data according to header information, resulting in a complicated transmission/reception process. Furthermore, the division method cannot maintain compatibility of a lower layer (i.e., a WDM/SCM-PHY), such that it requires a new physical layer. The tagging method for assigning a transmission sequence to the frame records such a transmission sequence to a header, adds the header to the frame, and transmits the frame with the header. However, the tagging method must reconstruct data on the basis of header information of the received frame, and must correct a standard protocol, resulting in a complicated implementation.
In conclusion, the above-described conventional methods cannot effectively use resources of the WDM/SCM-PON system.